The U.S. Environmental Protection Agency estimates losses of 60 to 80 billion kWh attributable to distribution transformer inefficiencies, which rob U.S. business and American consumers of approximately $4 billion per year. Currently, there are two basic arrangements for the iron-cores used to build distribution transformers: (1) “Core-Type” having cores assembled by stacking laminations and the transformer is completed by sliding pre-made windings; (2) Shell-Type having a continuously wound core that is cut and wrapped around the windings a few laminations at a time. In both arrangements, the finished core has air gaps that increase the magnetizing current and the no-load losses.
Toroidal transformers are not presently in use in power distribution systems. Toroidal transformers have typically exhibited unacceptable failure when subjected to the “impulse test”. To assure the quality of the insulation system, all utility-grade pieces of equipment should pass the lightning impulse test, among other tests. This test is performed in high voltage laboratories and consists of applying a set of lightning strikes of a given intensity and shape to the equipment under test. In the case of a distribution transformer, even one rated at 2.4 kV, the applied lightning impulses are of 95 kV. This test serves to give confidence to utilities that the transformer will not fail at energization or on the first electrical storm. Given the lack of experience with toroidal design at medium and high voltages, efforts have been made to develop the technology to pass the impulse tests as well as study the thermal performance and produce a sound mechanical design. Some of the design issues that have been solved include: Impulse response, matching the specification of leakage impedance, and thermal analysis. Reported problems with previous medium-high voltage toroidal transformer designs include failure to pass the impulse test, a low utilization factor, and the destruction of the core during the short-circuit test due to the strong electromagnetic forces.